Minimum shift keyed (MSK) modulation has received considerable attention over the past decade due to its potential as a bandwidth and power efficient constant envelope modulation technique. Two generic techniques for the modulation and demodulation of MSK signals are referred to as the parallel and serial methods. Both are completely equivalent in terms of bandwidth occupancy and bit error rate (BER) performance.
The parallel method amounts to quadrature multiplexing half sinusoid pulse shaped data streams staggered by one-half symbol period on quadrature carriers. Practical implementation of MSK modems employing the parallel approach require close balancing and alignment of the in-phase and quadrature channel data signals on carriers which are themselves balanced and in-phase quadrature. Similarly, at the receiver, careful balancing and maintenance of phase quadrature is required to minimize distortion and, in particular, cross talk.
On the other hand, with the series approach, the MSK modulated signal is produced from a bi-phase modulated signal by filtering it with an appropriately designed conversion filter. The problems of balancing and maintaining phase quadrature carriers of the parallel approach are therefore replaced with the task of building the conversion filter which, ideally, has a sin (x)/x frequency response.
Various approaches have been taken to building serial MSK conversion filters, including the use of surface acoustic wave (SAW) devices. Specifically, see F. Amoroso and J. A. Kivett, "Simplified MSK Signaling Technique", IEEE Transactions on Communications, volume COM-25, pp. 433-441, April 1977. Such devices are limited in bandwidth to roughly ten percent of their center frequency which is, in turn, limited to a few hundred megahertz. Thus, the maximum data rates of SAW implemented MSK modems are inherently limited to well under 100 megabits per second unless special fabrication techniques are used.